Shear Key Design 2

[mgt89]

Greetings all,

I need to design a base plate connection for a column that is subject to high values of shear. I´m having trouble finding a reasonable solution by only accounting for friction+shear on anchor bolts. For that reason i will need to use a shear key welded to the base plate in order to increase the resistance of the connection.

My question is: do you think it is acceptable to account the contribution of the friction + shear on the anchor bolts + resistance of the shear key on the calculation of the shear resistance of the connection? Or do i need to only work with friction+shear on anchor bolts or friction+shear key


Thanks a lot for the help.

 

[JAE]

Shear key only and don't count on anchor bolts at all.

Or you can embed plates into the top surface of the concrete and use welded vertical tabs on the sides of the base plate to take the shear down into the embeds.

I think AISC's stance is to minimize shear on anchor rods....we use them all the time but when the shear is high we go with an external load path instead of the bolts.



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[mgt89]

Hi thanks a lot for the answer. So i may only use the shear key resistance. But i can always account for the friction created right?

Can you elaborate on the second method that you described? English is not my first language and i did not understand it very well.

Thanks a lot.

 

[JAE]

See if this makes sense:

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[Leftwow]

Dumb question here. What can cause this much shear?

 

[wannabeSE]

What can cause this much shear?

An earthquake can be one reason

 

[hokie66]

Wind is another.

 

[KootK]

Heck, a light breeze can exhaust the shear capacity of an anchor bolt only connection if the pier is small/reasonable in size.

@JAE: great detail. I've used a version with a single, solid embed plate below the the column which is inferior from a concrete placement standpoint. This is much better. There is something about this style of connection that I've had a hard time resolving however. It seems to me that, despite the fact that the shear and tension reside in different anchors, all of the anchors contribute force to the same concrete breakout cone. How have you been handling that?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

In some cases the headed anchors (and embed plates) that we show have to be stretched out over a longer distance.
In one case we used steel tubes stretching between columns with anchors along the tube length. This works well except where the design includes doors through the wall.





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[KootK]

Thanks JAE. It just annoys me that, when I put the the headed anchors where they naturally want to be (near the base plate), I find myself without a robust method for evaluating capacity. When it comes to shear + tension, divide and conquer seems to be the name of the game.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[delagina]

am i the only one who doesnt like JAE sketch. why the need to have field welding. you could have shear key at the bottom of base plate, shop welded.

 

[JAE]

delagina - the shear key idea is fine with me. I'm just always concerned about the contractor getting the slot lined up correctly with respect to the anchor bolts.

Plus getting good fill with non-shrink grout around it is a concern as well.

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[wannabeSE]

JAE
I like the concept. But, I am not thrilled with the one-side fillet welds on the shear tab plates with prying/bending on the weld axis. But there many are ways to alleviate my concerns - calc the weld to justify or modify the detail a little.

 

[JAE]

One other thing that just occurred to me on the shear key. In light of the ACI 318 Appendix D provisions, I'm not sure what sort of "valid" calculation there is on a shear key.

What I mean is, there is surely a method that has been suggested, promoted and used in our industry for calculating how much shear key is needed....or conversely...what a specific shear key with depth, width, etc. provides in terms of capacity. However, under App. D our world has been turned upside down by the much lower capacities provided today vs. older bolt tables and capacities previously used.

I am just wondering if something similar applies to shear keys as it is a sort of "wide" bolt breakout that would occur with large shear forces in a formed key. Especially if the column has uplift on it during major shear events...i.e. no confinement of the concrete in front of the key.



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[KootK]

Adding to JAE's sentiments: Link

See this thread for my version of JAE's detail that I use when conditions permit: Link

I like the concept. But, I am not thrilled with the one-side fillet welds on the shear tab plates with prying/bending on the weld axis.

Is this still a concern given that the shear would be directed out of the page?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

KootK - appendix E of your first link shows some shear key capacity methods/calculations.

However, the paper summary cautions that these results, tests, and recommendations do not include enough tests to establish proper phi factors for use in design - only raw formulaic equations for Rn capacities.
I guess you could still use 0.75 or something but you'd be on your own with that.

But they do seem to approach the shear key design similar to Appendix D in ACI 318.



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[wannabeSE]

Thanks Kootk - for some reason I did not look at the orientation of the column in JAE's sketch. I assumed the shear plates were perpendicular to the column web. My bad.

 

[WillisV]

I skip the bottom weld entirely and just use post-installed angles welded at the top to the baseplate and screw anchored through the outstanding leg to the concrete. Eliminates embeds. Only upper weld required. I design the upper weld for the full eccentricity and the screw anchors for shear only. I keep the angle to the minimum thickness possible to allow it to bend somewhat and let the main anchor rods do their job in tension.